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Would it be possible for one airliner to tow another?
Comedian/podcaster/QI elf Dan Schreiber asked on Twitter, as proxy for his 4-year old

if it would ever be possible to hook a plane (like a 737) onto the back of another plane, like a train carriage, and then have both take off?  I know the answer is no, but just wanted to double check with you, Twitter

and continued

I knew about gliders...Just wondering about the Boeings...in case some one comes in with an XKCD-style answer

For the purposes of focusing our question, I'd like to ask:
can a jet airliner TOW (not piggyback / eat) another airliner from takeoff to landing?
It is pre-noted that this is not a good idea, voids the warranty, etc.  However, laying out considerations / difficulties / show stoppers, with particular attention to modern flavors of the 737, or noting which if any airliners would be preferable tow-ers or tow-ees, will be appreciated.

Particularly related answer here https://aviation.stackexchange.com/a/8952/3146

An F-102 or F-106 has been towed by a long cable attached to a C-141 in a NASA test project.

Another case of you COULD but why would you want to?

@CarloFelicione it was originally asked by a 4 year old!  Little wonder that it isn't a practical idea!

@Paul this question is not about piggyback.  were any of the early shuttle tests tows?

@quietflyer I was trying to exclude carrier aircraft that entirely contain the carried aircraft.  In practice that's rare and typically the carried aircraft has to be partially reassembled after.  Eat just seemed like a fun way to put it.

Pic source
Yes it should be possible for an airliner to tow another airliner. In WW2 gliders with paratroopers on board were towed to their drop zones, the technology has been around for a while.
Another interesting story can be found in this link, about capt. Robbie Risner using his F-86 Sabre to push his wing mate's F-86 into friendly airspace during the Korean war. A fighter jet pushing another fighter jet.

Risner told Logan to shut down his engine, now almost out of fuel. Then he gently inserted the upper lip of his air intake into the tailpipe of Logan’s F-86....Miraculously, Risner nudged Joe Logan’s F-86 all the way to Cho Do, maintaining an airspeed of 190 knots and enough altitude to stay out of range of automatic weapons.

One airliner pulling another one would indeed take structural preparation. The wing/body intersection is the strong point of the plane, dimensioned to absorb the forces from the jet engines which will now be provided by the tow ropes. If the tow ropes are attached at this point pitch moments will be produced into both the towed (nose up) and the towing (nose down) aeroplane, which receives an automatic trim correction for the engine thrust.
Issues to tackle:

Stability of the towee.
The towed plane will need to deal with interference and stability issues, and will need to have its flight controls powered. The B737 does have (very heavy) manual backup modes for the elevators & ailerons, the secondary flight controls require electrical/hydraulic powering. From the APU or from a RAT (which the B737 does not have). The batteries provide 60 mins of backup power, but powered flight controls are essential.
Longitudinal stability should be OK, since the plane is built to deal with the thrust of the turbofan engines. Directional stability must be improved by leading the tow rope through a support fixed at the node wheel structure. The rope won't always pull straight ahead like the engines do, and this degree of freedom needs to be eliminated as much as possible.

Wake turbulence. The towed plane will need to evade wake turbulence from the towing plane, as depicted above. So the towed aeroplane can take off first, and remain above the vortex field - or fly below the wake field as in this video. As @Neil_UK mentions in a comment:

Wake turbulence is not so much a problem, as a feature, for sail-plane towing. Once reasonably competent at straightforward tow-launches, the next exercise is to 'box the tug', that is, flying in, under, over, left and right of the tug plane's wake turbulence. –
Neil_UK

And on the strength of the wake field, from the FAA site

Weight, speed, wingspan, and shape of the generating aircraft's wing all govern the strength of the vortex. The vortex characteristics of any given aircraft can also be changed by extension of flaps or other wing configuring devices. However, the vortex strength from an aircraft increases proportionately to an increase in operating weight or a decrease in aircraft speed. Since the turbulence from a “dirty” aircraft configuration hastens wake decay, the greatest vortex strength occurs when the generating aircraft is HEAVY, CLEAN, and SLOW.

So the towing plane can reduce the wake vortex by eliminating the payload, extending its flaps, and speed up to max climb speed (point 3. below)

Optimum range.
The towing plane needs to provide thrust to overcome drag of two planes. The towed plane has its own wings and provides its own lift, so we'll only need to consider increased drag on the towing plane.
In the Breguet range equation $\frac{V}{sfc} \cdot \frac{L}{D} \cdot log \frac{W_i}{W_f}$ the towed/towing plane combination has twice drag D, and the corresponding speed for maximum range will be lower than for a single plane.

One thing is clear: it can be done, because it has been demonstrated already: the Eclipse project mentioned in @ErinAnne's comment, a Starlifter towing a Delta Dart on a 1000 ft rope. This report mentions how the attachment point on the Dart was constructed at the nose, how the towing took place successfully (the Dart pilot took his hands off of the controls), and how the wake turbulence introduced some shakiness so stay either below or above the wake field.

excellent point on the introduced moments and I also really appreciate the link to your answer on the 737's flight controls

FYI I expect your answer will be the accepted one but I'm going to activate the bounty once that's allowed so the acceptance will be delayed a bit by the 2-day ask-to-bounty time and then letting that stand for a bit longer

There was also a case of an F-4 Phantom pushing another F-4 during Vietnam using the tailhook: https://en.wikipedia.org/wiki/Pardo%27s_Push

I tried to track down more info about the Eclipse C-141A-towing-F-106 demo and, interestingly, in that the C-141 took off first and the F-106 stayed below.  Haven't found an actual report yet talking about their wake turbulence conclusions, but https://www.f-106deltadart.com/eclipse.htm goes the deepest into the wake turbulence evaluation that I've seen.  https://archive.org/details/NIX-EC97-44357-13 shows a good video of one takeoff

@ErinAnne Indeed, the F-106 stays way below the wake turbulence field, which is possible as well. The F-106 needs more speed than the tow plane to get airborne, when it lifts off it does so at a high angle of attack - below the turbulence field is best for it.

Here's a quick take that I will attempt to evolve into a better, more-XKCD-style answer over time:
I believe it's possible, but planning is required.
Trying to do this with fully-loaded planes isn't a good idea; you are more likely to succeed with reduced fuel (perhaps no fuel in the towed plane) and cargo.
This gives you excess thrust available in the towing plane, and every pound less of lift saves some fraction of a pound in towing force (I'm having trouble finding 737-specific numbers right now but Lift / Drag ratios of 15 to 20 aren't uncommon for an airliner and would change over the members of the 737 family).
The towing plane will have to generate both the thrust it needs and the thrust the towed plane would need, plus some additional from the towing bridle not being perfectly straight and taut.  For 737s specifically this gets a little iffy just based on the ratio between maximum takeoff weight (MTOW) and empty weight; I don't see any model of the 737 with that ratio as high as 2.0.  Perhaps you'd need to reduce the weight of the towed aircraft even more than "empty" (i.e. by removing engines).
There is every reason to believe that the towed aircraft could be rigged in such a way to receive the force, because the engine pylons and wing boxes already have to resist that force and more.  Presumably we will be towing at speeds slower than cruising speed to buy ourselves less overall drag and better Lift / Drag ratios.  Rigging the towing aircraft may be more complicated; my first guess would be attaching cables to the wing box at the wing roots, but that's a guess.
It will help if the towed aircraft can remain under control to maintain a good following attitude, use additional lift controls like slats and flaps during appropriate phases of the flight, retract and extend the landing gear, etc.  It is not clear to me at this time if a towed 737 would have to have a running engine to do so, or if an APU would suffice, or if newer models (or other aircraft like a 787) could run their controls entirely from an electrical connection from the towing aircraft (which could end up being one of the costliest things to engineer in this whole proposal).
I still don't see any reason to believe that particularly long runways should be necessary.  Though a 737+737 combo will likely not be able to operate out of all the runways a single 737 would, part of the point of long runways is to be able to abort takeoffs on the ground and we'll assume that among the operating controls that the towed aircraft has, it has working brakes.

Having done a bit of towing in my time I can attest that the towed aircraft will typically have a tendency to ‘lock out’ in any direction, i.e. the pilot of the towed aircraft must constantly make corrections to avoid going catastrophically out of alignment.  Whether a 737 is agile enough to be able to do that is a question in itself.

How long would the tow line be for the tower plane to avoid the towing plane’s wake turbulence? And is there a runway long enough to deal with that?

What about stability? One would imagine towing would inherently be more stable, but is that the case? E.g., can it start to swing from side to side (or up/down) because it wasn't designed for towing? E.g., at different speeds.

@StephenS Wake turbulence will be very bad and there is nothing that you can do to avoid it. This has been extensively researched, read about it here: https://en.wikipedia.org/wiki/Minimum_interval_takeoff
You will need a towing cable of several kilometres which is obviously not feasible.

@StephenS that's where all those circular runway proposals come into play!

There are runways out there which are over 4km long which were intended as fallback landing sites for the Space Shuttle program. Perhaps one of those would serve the job!

@mmomtchev: "and there is nothing that you can do to avoid it" - Sure there is, just have the towee fly above and behind or directly behind the tower rather than below and behind it.

@mmomtchev I don't understand why you think that ancient B-52s demonstrating that they could absolutely take off through multiple others' wake turbulence while also flying through dense black smoke means that for two aircraft you'd need not to do that and instead use a towing cable kilometers long.

B-52s need to be spaced at least 9 seconds apart - at 140 knots this is 630m. Once in the air, you will surely be flying faster than this. I don't know if the 737 handles turbulence much better then the B-52.

takeoff and landing are the most critical wake-turbulence phases.  In flight you have more options for both avoiding the turbulence and the ground.

A fully-fueled airliner needs to be able to take off on a single engine if one fails during takeoff. This indicates that every (2-engine) airliner has at least twice as much thrust as necessary to take off, fully-fueled (if only marginally); therefore, a fully-fueled airliner with two functioning engines should have enough thrust to fly itself plus pulling another airliner behind it.

@Skyler that's an interesting point.  I guess it'd vary aircraft-to-aircraft, but presumably engine loss on takeoff puts the engines into some kind of emergency operating regime?  Or is it common for takeoff thrust to be maximum available on a large airliner?  I know I've seen at least a couple aircraft where TO isn't quite all the way to the stop.

@Ruadhan2300 white sands is over 10km!

@Skyler And what happens if an engine fails during takeoff while towing?

@ErinAnne Two engine TO thrust is often well below max to save fuel. Crews will calculate the ideal thrust based on weight and runway length. If an engine fails, you push the other throttle to max. And at some airports, there are special “escape” routes that must be used because on the normal routes you’d hit terrain with single-engine climb rates.

@StephenS I was only looking at whether it's physically possible, not whether it's a good idea.

@StephenS presumably if an engine fails during your towing takeoff you release the towed aircraft and then continue with your typical emergency procedures.  But Skyler's right that's it's not supposed to be a good idea, per the stated caveats in the question.

It is surely possible in theory, but not that much in practice - at least not without some serious custom engineering.
One big problem is going to be find a point strong enough to attach the towing cable on both planes. Planes that are made to tow / be towed have special strong points.
Also do not forget that when towing, the towed plane usually releases and it lands on his own. This means that you will also need a cable release system and the towed plane will have to have everything required for autonomous flight and landing.
And as someone remarked in the comments, the wake turbulence is going to be extremely, probably prohibitively bad. It is already a problem for much smaller towing aircraft when launching gliders. However if the cable is long enough it should be possible to fly slightly above or slightly below the worst turbulence while keeping the tow angle close to zero.
Towing is very widely used for launching sail-planes and some heavier hang-gliding wings.

Are attachment points really that much of a problem? The engines (and by extension the wing spars) and the landing gear are made for those kind of forces. Maybe even the the tail (of the towing aircraft) and front landing gear (of the towed aircraft) would be strong enough.

When airplanes get towed on ground, they get towed by the front landing gear. But for take-off you will need extreme strength - equal to the engines thrust which for a 737 would be 7t to 10t depending on the model and the engines. The wing spars would probably be enough - but there are no such points exposed there on a stock airplane - you will need substantial modifications.

Ehm yeah, engineering. Engineers find the issues, and then fix them.

Wake turbulence is not so much a problem, as a feature, for sail-plane towing. Once reasonably competent at straightforward tow-launches, the next exercise is to 'box the tug', that is, flying in, under, over, left and right of the tug plane's wake turbulence.

@mmomtchev: Maybe find a very long runway to allow takeoff with a lot less thrust.  It would already need to be quite long for the engines of one airplane to accelerate the mass of two, so maybe imagine some very very long straight section of highway, or a custom built runway since this is purely hypothetical anyway.

Is 90 miles without a curve enough?  Australia's Eyre Highway even has turnouts for smaller craft to land comfortably.  But it might not be wide enough for an airliner.

If we're towing, why are we hauling the weight of fuel and engines on the towed aircraft? So, then we're building a whole new 737 size glider? Good luck getting that past the finance department.

the C-141 towing an F-106 mentioned elsewhere actually serves as an interesting example re: wake turbulence.  The C-141 should easily be generating wake turbulence comparable to that of a 737 (MTOW much higher than any 737 in existence) and the much lighter F-106...doesn't even seem to bounce around https://archive.org/details/NIX-EM-0008-01


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How would gyroscopic effects affect push-pull configuration vs conventional twin engine in the event of engine failure?
This answer, and the Wikipedia article on push-pull configuration both claim that aircraft with push-pull configuration are easier to fly in the event of engine failure, because the thrust provided by remaining engine would remain in the centerline.
However, lot of aircraft with push-pull configuration seem to be relatively small ones, so I am wondering how gyroscopic effects would affect the handling of the aircraft in the event of engine failure.
I found this answer comparing WWI era FB19's to F16's. But as I said before, lot of push-pull reconfigured aircraft probably have less mass and much slower than F16, so the calculations in that answer may not apply.
So my question is, is there any noticeable handling difficulties in push-pull configured aircraft, when an engine fails?

I'm confused @Krumia, are you asking about purely gyroscopic effects, or other effects? If it's other effects you need to make that clear.

@GdD: I'm asking specifically about gyroscopic effects.

Gyro precession effects from a propeller are minimal when at flying speed and making modest pitch and yaw motions, where the precession is mild and there is lots of aerodynamic damping.  You don't really notice it. Propeller slipstream over the rudder has a much bigger effect, which a lot of people may confuse with precession.
You have to make large sudden pitch or yaw motions at low speed, like when taking off a taildragger with a very large propeller. If you raise the tail suddenly at low speed, precession of the propeller will make the airplane swing noticeably.  Or, with aerobatic airplanes that make extreme pitch and yaw motions and where precession will have an effect that has to be taken into account (as well as putting massive stresses on the crank at the prop flange).
For wing mounted engine twins, the effects of offset thrust are the main limitation, with a minimum control speed that is usually well above the stall and below which the airplane will roll over.  Centerline thrust aircraft avoid this problem for obvious reasons.  In any case, I don't believe gyro precession is a factor at all in a Cessna 337, on either engine, or on a regular twin for that matter because the pitch and yaw rates you generate aren't enough to create significant precession effects that could affect controlability.
In the end though, with piston twins, gyro precession is the least of your problems.  The biggest one is the fact that the loss of 50% of thrust leaves you with almost no performance reserve.  Wing mounted engines leave you with a somewhat more dangerous situation due to the ability to lose complete control well above the stall by dropping below minimum control speed, but even with center line thrust, the single engine performance is so marginal that if you are at gross weight and above sea level and it's hot, you are going down anyway. Yes there is no Vmc limit, but you are still faced with having to maintain minimum speed for performance and chances are you are still going to crash land eventually and you may ending up stalling and spinning it the regular way if you panic trying to will the plane to fly, just like a regular twin.
For this reason I don't believe center line thrust aircraft have all that much better a safety record following engine failures than regular twins.

Aren't all multiengine aircraft required to have powerful enough engines to provide them with a large performance reserve even in single-engine flight?

The large performance reserves of jets are largely a function of the huge amount of power required to go fast so that 50 percent of thrust is plenty for a decent climb.  Turboprops are kind of in between and have passable climb on 50 percent thrust.  Piston aircraft don't have as large a speed range and 50 percent gives maybe only a 5-10 percent margin over the minimum required for level flight.   A DC-3's single engine rate of climb is 300 fpm at gross - at 59F at sea level.  A piston twin that can do 500 fpm single engine at gross is a pretty high powered machine.

One could look at the NTSB records of the Cessna 337, a front & rear engine push-pull 6-seat aircraft, vs that of another comparable sized twins with engine mounted wings, such as a Piper PA-30, and try and draw some conclusions.

That would be a useful exercise.  My own belief is from some long ago articles in Flying Magazine where they used to discuss multi engine safety a lot, with the most interesting thing being that from pure statistics, a single is actually safer than a low powered twin from a "chance of dying" perspective.  Anyway, they had written about the 337 and how it didn't seem to have an accident rate much better than any other twin.  Similar to the Ercoupe, the stall proof "safety plane" that ended up with no better an accident record that most other light planes.

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What if you refuse to fasten your seatbelt?
As a passenger, what happens / can happen if you refuse to fasten your seatbelt when you are told to, either during take off/landing or (bad) turbulences?
I've recently read a Facebook post that supposedly described such a situation. It didn't give many details, so that's why I'm here.

Just out of curiosity, why would you not want to fasten your seatbelt?

Except in unusual circumstances, an airline captain has a professional responsibility to not take into the air any potential problem he is aware of, and generally speaking the presence of a potentially disruptive passenger will be brought to her/his attention.

@Skip Miller: the same reason you wouldn't want to wear a mask during the Covid pandemic? I'm not condoning either of these actions, I just wanted to say that people for unfathomable reasons sometimes simply do things that are not in their best self-interest...

Especially in the US this would be taken very, very seriously.

In the US it is an offence under federal law.
A loose 180lb/80kg projectile inside an aircraft is a threat to other passengers during turbulence, in the worst cases of passenger misbehaviour cabin staff can and have forcibly restrained passengers and/or diverted to a nearby airport to have the passenger deplaned and sometimes arrested.
Seven Ways to Get Kicked Off a Plane
Family kicked off plane

A 3-year-old, who refused to use his safety belt, caused a flight to be delayed after his father could not get him locked in his seat belt on board an Alaska Airlines plane.
The pilot returned to the gate on Saturday and asked the father and child to get off the plane.

Why you need to wear a seatbelt

For passengers who refuse to wear seatbelts when the sign is illuminated and who are disruptive to the crew, the FAA can impose a maximum fine of $25,000.

The fatalities in the Asiana 214 crash landing in San Francisco on July 6, 2013, were not wearing seat belts during the accident. One person suffered fatal injuries from being thrown across the cabin when the tail struck the seawall in SFO, and the other person fell out of the plane and was run over by a rescue vehicle. It is really stupid not to wear seat belts.

It's an offence in a UK registered aircraft also.

To be clear the underlying federal offense is not specific to seat belts it generally covers any refusal to comply with aircrew instructions or hazardous behaviors, and the judge can lock you up in place of the fine.

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Why does flight training usually start with (unsafe?) propeller planes and not with (safer?) jets?
I live in the UK and would like to learn flying jets, but most courses seem to start with propeller planes.
Not sure if this might be a misconception about the safety of different aircraft types, but I remember having read or heard somewhere that small propeller planes are, well, less safe than jets; maybe because they are more fragile and less powerful than jets when it comes to bad weather and are also difficult to recover from dangerous situations... not sure and cannot find the reference for now.
But also doing a quick search for popular aircraft types, the propellers like Cessna seem to have huge numbers of crashes/incidents compared to jet engine aircraft.
So the question is - why are propeller planes favored in training e.g. for the Private Pilot License? Is it simply because they are cheaper and wouldn't be such a huge loss of they crash?
Are there any alternatives where I could start with a jet engine?

Why should a prop plane be less safe than a jet?

Wouldn't you expect that whichever type of plane is used most by inexperienced pilots would have the most crashes/incidents?

Don't know about the “recover from dangerous situation” bit but bad weather is something you can mostly avoid. Many accidents result from inexperienced pilots flying anyway because they want to get somewhere but if you are in training, you would simply not get in the air when it's dangerous (and your instructor should certainly be wary of that).

ICAO regulations require you to have a valid pilot license and an instrument rating (and thus plenty of previous experience) before starting a type rating on any jet. It is therefore not only illogical but also illegal for a school to offer initial pilot training on jets.

@Radu094 regulations can be changed, though of course bureaucracies tend to be very slow to do so. And the better the reason to change them, the slower they tend to be.

Safety is not an inherent property of aircraft size or engine type (large vs small, piston vs jet). Consider that smaller, simpler aircraft are often manufactured in larger numbers. Pilots of the big, fast, complex machines spent years and thousands of  flying hours getting there; all low-time pilots (those most at risk of making rookie mistakes) are flying the smaller, simpler, easier to fly planes. Rather than drawing conclusions about safety from gross statistics, examine some specific incidents and their causes.

Would you learn to drive in a Ferrari? In some ways they're safer than a Micra, or Focus but just not practical for new drivers.

To the question of whether jets actually are safer, does anyone have per hour accident statistics for non-commercial jets?  It certainly seems that there are a relatively large number of private & military jet crashes, but that might just be that they're more newsworthy than a crashing C-150.

The safest option is NO motor at all.

A similar quandary exists in the world of radio-controlled model airplanes.  I always advise interested beginners to stay away from propeller-powered models if at all possible, and to purchase an "electric ducted fan" model, or a better yet, an actual turbojet-powered model.  Because they are more reliable and safer.  Retrograde behaviors are acceptable in some contexts, but not if several hundred dollars worth of airplane may be on the line.

Light Propeller Aircraft, if flown within their design limits, are just as safe as jets. If you do a proper and thorough daily inspection, you are far less likely to suffer any form of mechanical failure in the air. In saying this, the majority of all crashes, be it in a Jet Powered Airliner or a Propeller Driven Light Aircraft are caused due to pilot error.
You start out learning in a light single as:

They are lighter, and slower and can usually take the beating handed out by inexperienced pilots, this being:

Hard landings
Slipping onto the runway
Just hammering the day lights out of the engine.

Jet Aircraft are extremely expensive, and generally fairly heavy. Inexperienced pilots generally pay AU$80,000 for a Commercial Pilot License (In Australia) which qualifies you to fly single engine aircraft not over 5,700kg under the VFR by day. This is an absolute, level entry qualification looking for a job (most people will find that they at least also need a night VFR rating or a MECIR), so to add the cost of doing training in a jet on top of that, even if it is just for PPL, will be massive.
Jet Aircraft will be insured to be flown by pilots who have an amount of total experience and experience on aircraft/engine type. So, again, unless you're willing to pay the massive excess and premiums to be put on the insurance, you're going to have to fly a prop.

Now, you have identified that a lot of crashes are light singles (i.e. Cessna). This is because generally, these aircraft are not flown with the same discipline as a Jet Aircraft. Also, the pilots flying them are generally lower hour pilots who do not have as much experience (again, this is a generalisation, not an exact figure of the total community).
As for starting in a Jet, I honestly doubt that there is any flight school on the planet that will allow you to start training in a Jet (Even Fighter Jet pilots start out flying tiny little single engine aircraft).

In the USAF and US Navy JPATS program the express wish in the specification was for a jet-propelled entry trainer. The idea was to train beginners on jets to avoid teaching concepts like rudder input on take-off to counteract prop torque. In the end, a modified PC-9 turboprop was selected. However, it does compensate for torque by computer control, so it flies almost like a jet.

@PeterKämpf - Really? - Australia still uses the old CT-4 to do basic training, and then moves onto Pilatus PC9's.

@PeterKämpf: Interesting.  The US military also operates a few types of prop aircraft, so I suppose if they had done this, they would have had to split trainees into "prop" or "jet" groups at the very beginning of training.

@NateEldredge not necessarily; currently they train everyone on prop and "convert" most people to jet later. Surely the reverse would also work - start everyone as jet and "convert" to prop as necessary?

I take minor exception to the claim that most accidents are due to pilot error.  The proximate cause may be pilot error, but in many cases, this error occurred because of poor controls layout, unrealistic operational sequencing requirements, and so on.  I recall an analysis (possibly John Denver) where the crash was due to pilot failure to open a reserve fuel tank (I think), but in fact the control lever was high and behind the seat, and a pilot trying to control a small craft w/ dead engine, in mountainous terrain, is likely to be fail to maintain attitude while reaching for an unseen lever.

@CarlWitthoft - Sadly, those are the statistics. About 80% of accidents due to pilot error, 15% due to mechanical failure, and 5% due to other issues. As a pilot, you can't blame poor layout of controls, as you should be familiar with the layout of your aircraft prior to flying it. We can't blame unrealistic operational requirements, as at the end of the day (and trust me, I know how hard this is) as PIC you have to make the decision on whether it's safe to fly.

There USED to be a plane that the USAF used for a certain group of trainees that was a small single engine jet.  The pilot students would start on this plane.  But the USAF has since move to prop planes for everyone.

The (Cessna) T-37 "Tweet", a twin engine jet, used to be the primary trainer for Undergraduate Pilot Training (UPT) in the United States. It was replaced by the T-6 about 8 years ago, a single engine turboprop (PC-9 as stated above).  The T-6 was considered superior with it's increased thrust-weight ratio and fuel efficiency. Additionally, before you enter UPT all USAF flight students needed to got through a flight screening process on a traditional introductory aircraft (C-172, DA-20, etc) - the T-6/T-37 were never the first aircraft flown by students.

indeed, JPATS (T-6, PC9) is NOT a basic trainer. It replaced jets for advanced training because it's cheaper mostly than using jets for that.

@jwenting Incorrect.  The AF and USN both use the T6 Texan II for primary (initial) flight training.  The T6 does not appear in the JPATS flight syllabus for any intermediate or advanced training.

In the hands of an inexperienced pilot, a jet is definitely the unsafer choice.
Due to the general higher speeds, there is less time to recover from errors and they require more anticipation to be ahead of the game. Small errors will grow into large errors in relatively short time. In slower aircraft there is more time to correct.
In addition, jet and turbine engines have slower response times than piston engines, which is important when you need to abort a landing for example.
The fact that there are more accident and incidents with propeller aircraft is telling little about the inherent safety of these aircraft, instead it says something about the relative inexperienced pilots that often operate them.

Small propeller aircraft have more incidents and accidents, because they can be flown by single pilot and that pilot is often not very experienced (PPL requires only 40 hours in US and 45 in Europe), while anything transport category requires two pilots and the pilot in command has to have ATPL and for that they need to have 1500 hours experience (yearly limit is 1000 hours). Note that transport category propeller aircraft like ATR-42/72 or Dash-8 don't have significantly higher accident rate compared to transport jets.
If flown by experienced pilots, a single engine plane is slightly less safe, because if the engine quits it has to do emergency landing and it may not always be possible over rough terrain.
On the other hand though, propeller planes are slower (propellers are efficient at slow speeds, jets only at high speeds) and easier to handle because jet engines react slowly to power changes. And smaller aircraft has less inertia, so it reacts to control inputs more readily too. Therefore you want to start with a small propeller aircraft in which mistakes can be more easily fixed, build up some experience with the general behaviour and only then progress to the faster and larger jets that require gentler handling and more thinking in advance.

I seem to remember (and don't have time to look it up right now to verify) reading a statistic where piston multi-engine aircraft were actually involved in more fatal accidents than piston single-engine aircraft after an engine failure.  Mainly because of improper procedures...  Are you sure about the single engine plane being "slightly less safe", or is this just conjecture (and trust me, it makes sense to mee, but I'm not sure that it's true).

He did say, "If flown by experienced pilots," which you might interpret as "someone well-trained and experienced enough not to make errors in the procedures that are more difficult in a multi-engine aircraft." But I think it's a very good point that multi-engine aircraft appear require more skill to fly than single-engine aircraft at the same level of safety. (Or so I have heard, as well.)

@Lnafziger you're right. In a twin, a somewhat common cause of a crash is (or used to be) to shut down the working engine after an engine problem.

@Hobbes, indeed. For a recent example see TransAsia flight 235. Even more common cause is failure to maintain speed above $v_{mca}$, or sufficient margin above it for turning. Still just a tiny fraction of engine shutdowns in flight ends with an accident.

Small piston airplanes are "cheap" (as airplanes go) to operate and jets are an order of magnitude more expensive.  For example, you can find small piston trainers on the order of \$100/hour to operate whereas a jet is often well north of \$1000/hr to operate.  If your wallet can accept this, this is just one barrier to starting training in a jet.
Small piston training airplanes are everywhere.  Go down to your nearest airport and you'll probably find a few.  Jet airplanes for training for the most part just do not exist.  The vast majority of jet training happens in simulators, and these simulator courses are really only designed to provide type ratings and possibly ATP ratings, not primary training.  This means you'll probably have to buy or lease a jet.  This will be very expensive.  This is your biggest barrier to starting training in a jet.
If you own or rent a jet, you want to insure it.  I sincerely doubt you'll find anyone willing to underwrite hull loss insurance on a jet used for primary training of an unrated pilot. Perhaps you could find coverage for a premium equal to the full replacement cost of the jet, but that really isn't practical.  This is a barrier to training, but if you can afford to do this you might not care that you can't find insurance.
Speaking of type ratings, it is very likely you are going to need one of those to operate the jet, this will complicate your training and checkride because type ratings are tested to ATP standards, which means you will be required to fly to a higher standard with tighter tolerances than you would otherwise be requires at this stage of your training.  You'll also be crippled without an instrument rating because you can't fly the jet above 18,000 feet (USA) without one.
Speaking of checkrides, you'll need to find an examiner that is typed in the jet you are using and who can conduct the checkride.  Easier said than done.  A typical jet isn't really suited to the private checkride either.  The required cross country navigating with a map and the required diversion would be a lot of work in a very fast and complex airplane for a brand new pilot.
Before you can go on a checkride, you need training.  You need a CFI who is typed in the jet and that CFI is going to be more expensive than the CFI you would be using in the piston airplane.  Probably much more expensive.   You will probably also need to spend much more time with the instructor than you would in the piston airplane because the jet moves much faster and you will be flying to ATP standards instead of private pilot standards.  More practice == more money.
If you want to be in a jet ASAP and you can afford to own one and insure it, your best bet is to just do the private in a piston airplane then do an instrument rating in a piston airplane, do your multi-engine rating in a piston airplane, and then get a type rating in the jet to transition into flying the jet.  You'll probably also need to hire a typed pilot to fly with you (for insurance costs) until you build a significant amount of time in the jet.  If you didn't buy a jet capable of single pilot ops, then you'll need to hire a pilot because the plane requires two pilots anyway, but the entirety of my post assumes you picked a single pilot capable jet.

What would be the point of buying insurance if the premium was the replacement cost? Why not just put that much money in a savings account?

There are a lot of good answers here already that cover some of the most important answers: the extreme cost difference, jet engines respond slower, very low availability of training equipment and flight instructors, inability to (legally) fly the jet at the altitude it's designed to fly at (since you need an instrument rating to enter Class A airspace, which is everything above 18,000 ft in the U.S.,) etc.
However, another important thing to note is that you can practice maneuvers in a small, single-engine aircraft that simply aren't practical to practice in most jets. In a PA-28, it's no big deal to practice stalls and stall recovery (which is a required skill for a private pilot certificate.) Practicing stalls in a Learjet, let alone a 737, is quite another story. Same for 60 degree banks, slow flight, etc. These are things that all pilots need to practice, but which just aren't practical (or safe) to practice in most jets.
As for jets being 'safer,' as others have said, the safety record of light aircraft is more related to the average skill level of the pilots usually flying them versus the average skill level of the pilots usually flying jets. An airline pilot will have thousands of hours of flight time. Many accidents in small, single-engine aircraft, on the other hand, are from low-hour pilots and even student pilots. A student pilot flying a jet will be decidedly unsafe. Student pilots are much safer when flying small, light, and (relatively) slow aircraft that are also not nearly as complicated to operate. The light aircraft are much more forgiving to the mistakes that student pilots make than jets will be.

Look at the ATP practical test standards (PTS) and you'll find most of the maneuvers you list to be part of the ATP checkride.  Steep turns (45°), straight ahead and turning stalls (to the stick shaker) and recovery with minimal altitude loss are all part of the ATP checkride, type ride and recurrent training.  Yes, that is typically done in a sim, but could easily be done in the real airplane.  In sim training slow flight is also covered in a jet (slow enough that the stick shaker is active but fast enough that the pusher doesn't activate).

True, but would you not agree that those are much more serious conditions in a jet than in something like a Cherokee? Requiring an ATP to practice something is quite different than requiring it from someone who's never flown an aircraft before. I suppose I should have clarified that it isn't practical for a student pilot, though, rather than just that it isn't practical.

Not really.  The only real difference in the jet is that you'll be doing to ATP standards and the maneuvers will be performed in IMC (no visual reference outside), which is not the goal of primary training.  A steep turn is a steep turn.  Once you know the airspeed for entry for your airplane and understand the need to add power to maintain altitude, it doesn't really matter what airplane you are in.

ATP checkrides are often done in light singles and twins, and occasionally sims.  Doesn't happen in real world large airplanes simply because it's too expensive to use them for something that can be done MUCH cheaper another way.

I've heard it claimed that small Cessnas are favoured by flight schools in part because they exhibit so many of the classic small-plane flaws in their handling.  And, while it's relatively easy to get into trouble in a Cessna, it's also relatively easy to get out of it again, as it is specifically designed to recover cleanly from unusual flying attitudes if the pilot applies the correct techniques.
That means that the new pilot is introduced to these flaws, and the correct way to handle them, in the relative safety of having an instructor sitting next to them with duplicate controls, being at moderate altitude and generally not moving very fast.  It also introduces an element of "if you can fly this, you can fly anything" - though that isn't literally true, as multi-engine aircraft, retractable gear and instrument flying are significant enhancements to difficulty as well as performance.
To use an automotive analogy, you shouldn't start a learner driver out in a 300hp sports car with an automatic or semi-automatic transmission, nor even in a 10-litre diesel-powered articulated lorry, even if he aspires to drive such a thing one day.  It's much more likely that he'll be driving an 80hp hatchback with a manual transmission (at least in Europe), because that's what's affordable and has got plenty of performance for ordinary driving.
The real point is that the small, piston-engined aircraft is a good place to establish the basic skills, which are then built on if the pilot decides to move up to more capable aircraft later.

There's also the benefit of lower maintenance costs for Cessnas & Pipers for the flight schools.

Gee, I don't know.  What are these supposed "flaws" that your classic flight-school Cessnas (152, 172) exhibit?

@quietflyer The propwash impinges noticeably on the tail, so you have to adjust the rudder position based on speed and power changes in order to maintain coordinated flight.  If you get into a stall while uncoordinated, a spin may result.  The aircraft as a whole is slow and draggy, and can't climb to high altitudes (nor very quickly in general).  Handling it in a crosswind takeoff or landing is tricky due to the low speed at those phases of flight.  There is usually no autopilot, so you have to hand-fly it.  All great for teaching, less so for owning.

A big part of learning to fly is learning to "stay ahead of the airplane". Know what the airplane will NATURALLY do on its own, versus what you WANT it to do. Things happen a lot quicker in a jet, so a slower prop is a better place to learn this skill. One thing that pilots must learn is emergency procedures such as engine failure, and in a prop plane you have more time to make a safe decision about a forced landing.

This is mostly true, except that the last part isn't true from cruising altitude. You can glide much longer and much farther from 30,000-40,000 ft at 400+ knots than you can from a few thousand feet at 100 knots. From the same altitude, though, you would have somewhat more time (but still probably less distance) in a light aircraft. Of course, your options for non-lethal places to land are greater with the light aircraft. Also, the odds of both engines failing at cruise on an airliner are much smaller than an engine failure in a light single-engine aircraft.

But you need to be IFR rated to fly above 18,000 feet, yes? And on O2.

@CrossRoads Either oxygen or a pressurised cabin, above about 14,000 ft.  The latter option also adds complexity to the aircraft, and failure modes which must be trained for.

I think there are time limits also, would have to look them up.

I have been a pilot for 10 years you learn on a prop plane mostly because of the speed. The faster your going the faster you need to think and react to not get "behind the plane" I would also have to say that in my opinion prop planes are safer less to wrong slower and fly at a lower altitude. And in addition to that in case you crash it less cost for flight school to replace

To learn to run, you must learn to walk.
You do not simply hop into a jet aircraft much less a multi-engine one. In fact, most US military pilots started out in single engine prop based aircraft, The T-34 Mentor and T-28 Trojan. These have now replaced by the T-6 Texan and T-37 Tweet.
Prop planes are better to learn in and tend to be significantly more forgiving than Jets. One of the primary reasons is that with Piston powered Props (Turboprops behave more like jets) you have a more linear response when making throttle adjustments. This is important when performing maneuvers, especially during a landing where you may be forced to wave off suddenly and need to immediately apply throttle to abandon the maneuver. They also tend to be significantly less complex, allowing a pilot to focus on actually learning to fly rather than having to be overly concerned with nuances such as "hot starts", "Flameouts", compressor surge, and throttle lag (as initially mentioned).
Additionally, single jet planes are nowhere near ubiquitous as single prop planes, meaning finding a single jet to do training in is pretty difficult without owning one. In flight training you're limited by your certifications. Most GA (General Aviation) jet aircraft tend to be dual engine, requiring a multi-engine certification. Additionally you can't really fly Jet aircraft at their performance altitudes without being sufficiently licensed to do so.
The main issue I take with your question is the insinuation of prop planes being inherently unsafe. They're not, and to imply such is dishonest at best. Where you may see GA Single Engine Props being more accidents has to do with an effect we call "Survivorship Bias" where statistics incorrectly lead to a conclusion that is misled and misinformed. This goes back to the days of old warfighting aviation where they examined war birds that came back and noted their damage. Naturally one would want to armour up the places that got shot up the most, but what a person considering that is simply not being aware of, is that the plane had returned safely with that damage. A significant discovery, that those obviously damaged places did not need armour and upon examining planes that didn't return, other more vital places should be armoured instead.
Answer:
Jet pilots simply have more time in the aircraft. They have to. You start out in a prop aircraft to learn the fundamentals, and work your way up, eventually learning IFR operations and Multi-engine certifications to get into a Jet. You get more experience. Jets are an extremely poor choice for beginners as they are prone to overcorrection and subject to poor flight judgement in an inexperienced pilots hands. Things happen fast, there's a lot to be learned with judgement and speed. It's not for beginners.
The majority of GA Accidents involve single engine props because most pilots are learning on them and tend to be generally less experienced. Not because jets are inherently safer. In fact, jets are inherently more likely to have serious, expensive problems; many of which are not recoverable in the air. Maintenance is also significantly more costly.

The T-37 has been retired for several years. The T-41 was for screening, not training. Air Force UPT pilots start out in the Primary (not "intermediate") trainer in the (turbine) T-6 or, in the past, the twin jet T-37, with as little as about a dozen hours in the screening airplane. And plenty of them learned to fly in those just fine. Prop time first isn't necessary, it's just cheaper.

@Ralph J, For what it's certainly worth, military pilots get significantly more training and seat time - so if anyone is to be trusted on their first out with a Jet, It would be them. Pilots in the military are selected for their acumen and ability. They tend to cut the chaff from the wheat. Kinda essential when they're being entrusted with pricey aircraft. Joe Q Public doesn't normally have this experience and discipline. Additionally, I did highlight that it is possible to train in a Jet, they're just not anywhere near common and do require more experience to fly competently in comparison.

@RalphJ, additionally thank you for the correction. I forgot about the T-6 and for some reason remembered the T-37. I have more memory of the T-34 because it's been prominent in most of my life (I'm not a military pilot).

Welcome to AviationStackExchange. While it can not be argued that military pilots are not the cream of the crop (which they are). I think the military pilots level of professionalism lies more in the military vetting process, indoctrination, culture, and mindset than it does in the amount of hours flown. When comparing military to civilian flight training, one must consider a civilian in a full immersion program could gain about 250 hours of flight time (not including ground and sim training) their first year. And, then go on to instruct over 1250 hours of flight time their second year.

One must remember that learning to fly is not just like learning to play a video game. A trainee pilot is expected to understand the aerodynamic behaviour of an aeroplane practically. The coordination of human reactions to the aerodynamic characteristics of the trainer aircraft is important, therefore good trainer aircraft are much sought after (that is why we had some legendary trainer aircraft like DH Tigermoth for example). Furthermore, at low subsonic speeds aircraft truly fulfill all the traits of a heavier than air classical aerodynamics. A propeller-driven slow-flying aircraft is definitely better, cheaper, more forgiving to a new pilot's mistakes. A pilot with initial training on a propeller-driven aircraft can easily graduate into flying jets.

There are probably more incidents with propeller aircraft for a few reasons.  First jet pilots in the USA will have more experience (I concur with other posts that experience is a primary factor in this case).  Certainly, propeller aircraft have limitations when compared to jet aircraft (less power).  However, kept within their capabilities, propeller training aircraft are very safe.  I would expect any pilot to agree that a small Cessna will more readily glide than jet aircraft.  This learning to take off and land in a prop helps the student pilot get a feel for flying- a feel for the fluid force of air over the wings, nose, and tail.

Prop planes have a longer glide ratio. (they fly farther when the engine quits. ) My father would have said jets have the glide path of a rock, the slower they go, the faster they come down!  Jets are also much more complex and sometimes simple is safer to maintain as well as fly.

Hello, and welcome to the Aviation Stack Exchange! I would guess that complexity is a big part of why people learn to fly in prop planes - feel free to elaborate on that! - but note that large transport-category aircraft tend to have much larger glide ratios than trainers: http://aviation.stackexchange.com/a/3001/136

Even if the point on glide ratio was correct (see previous comment and the answer linked therein), even 10:1 from FL300 still gets you farther than even 20:1 from 5,000 feet. And as the linked answer mentions, the ratios are more likely to be reversed; 20:1 from FL300 or 10:1 from 5,000 feet.

authors: All Іѕ Vаиітy

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Can radars detect all planes, including military ones?
This is with reference to a News Article that I read today. I find it scary that radars would not be able to detect an aircraft. Is it really possible? If yes, how can ATCs manage this situation?

Note that even non-stealth military aircraft can collide with civilian aircraft.

Reading through http://avherald.com/ I've got the impression that Indian news outlets, including Times of India, are rather sloppy checking facts when reporting aviation events.

Some military aircraft reflect little enough radar energy to be effectively undetectable. Note that the actual details are likely classified, but even the modern F-22 still has a radar cross-section of about the size of a steel marble. Still, the B-2, an older stealth aircraft, is claimed to disappear from air traffic radar when in stealth mode.
The same article on the B-2 also notes that it has "deployable radio antennas for navigation in civilian skies". When flying during peacetime and outside of an MOA, military aircraft generally have some level of coordination with ATC. If the pilots of a stealth aircraft don't want to be seen, then presumably they would be responsible for maintaining separation from other aircraft, as they cannot be tracked on radar. Of course, there's also plenty of airspace where aircraft don't need to talk to ATC at all (E and G in the US), and the pilot of each aircraft is responsible for maintaining separation.
That said, there are some things in this article that are unclear:

The Etihad aircraft received a TCAS alert for an aircraft that didn't show up for ATC. TCAS intentionally uses the same frequencies and signalling as ground-based secondary radar, so if an aircraft receives a TCAS response it should also show up for ATC. The article does note that the aircraft were "450 to 480km from Mumbai's coastline", so it's possible that the mystery aircraft just had a weaker TCAS response that wasn't received by a distant ground-based receiver.
The article suggests that fighter aircraft don't fly as high as 36,500 feet. A ground attack aircraft might not be able to fly that high, but most, if not all, air superiority fighters have service ceilings of at least 50,000 feet.

Thanks for the explanation! But as you rightly suggested the article does not bring facts well.

Stealth doesn't make aircraft undetectable, it only reduces detection distance. If the stealth aircraft leaves the transponder on, it is perfectly visible to ATC.

This is possible, and actually fairly common and expected.
Primary radars, as you most likely know, works by picking up reflected EM waves from an object. But many people do not realize that in engineering (as opposed to science), things are never perfect. Radars always pick up "ghost images", caused by reflections of clouds in the atmosphere, a vessel, terrain etc. I've yet to seen a raw radar image that has absolutely zero noise.
Moreover, in reality a plane is not a infinite small point in space, but a large object which reflects a certain amount of rays. These rays form a "patch" on the scope, and depending on the plane's orientation, this "patch" varies a lot.
If this sounds abstract, think of recording with a microphone. Even professional studio recordings has noise, and it is present in every single take. They are digitally suppressed in post production.
Likewise, a radar has to be tuned to "see" the specific type of reflections it is built for. In the case of civilian ATC radar, it is tuned to see large and slow moving objects, and filters out "noise" data which does not correspond to this profile. A fast moving military jet may be invisible to radar. A fast moving missile is more than likely invisible to controllers.
Partly due to their inaccuracy, controllers seldom use primary radar to track planes. They use "secondary radar", which in simple terms receives data like location, airspeed, heading and altitude broadcasted from the plane's transponder.

As for the article you linked, it states wrongly that fighter jets cannot fly at 36,000 feet, as pointed out by NathanG. Even propeller planes can fly at 25,000 feet nowadays. I also agree that probably the aircraft has a week ADB-S transponder that is not received by ground stations.


text:
Justification for the 2D cut-out method for determining a rocket's center of pressure
I've seen several tutorials on rocket stability where one can empirically obtain the center of pressure by cutting out a 2D imprint of the rocket and balancing the 2D shape as seen in the pictures below:

What is the physical/mathematical justification for this, otherwise, extremely simple method for determining center of pressure?  What are the underlying assumptions behid this method?  In what cases is it applicable?  When it is applicable, what order error should I expect from this method?  In what cases is it not applicable?

I think this assumes that the CP is the same as the CG...

@RonBeyer: The cg can be different on the vehicle compared with the 2d cut-out, since weight in the 3D vehicle can be distributed unevenly.

In physics this is known as argument from symmetry

I couldn't justify this method more simply than NASA does:

For model rockets, the magnitude of the pressure variation is quite small. If we assume that the pressure is nearly constant, finding the average location of the pressure times the area distribution reduces to finding just the average location of the projected area distribution.

So the pressure, constant everywhere, "cancels out" leaving you with only area.
And the simplification from a 3-D shape to its 2-D projected area is justified by the shape's relative simplicity and symmetry.  This wouldn't work with a grand piano.

Is axisymmetry a prerequisite?

Reducing axisymmetry makes the projected area depend more on the direction of projection, thus increasing the chance of a poor choice and thus the error of estimation.  Even in this question's illustration, the fins introduce asymmetry: if you instead projected along the fins' X instead of their +, then they'd contribute less, and the estimated CP would sit closer to the nose.  This method also can't notice if you have six (or a dozen!) fins instead of four.

This makes me wonder how much of a deviation from true CP is accumulated with each additional fin.  I’m guessing there is no heuristic n-fin correction to this method, is there?

When a fin's angle of attack is near zero, the aerodynamic pressure (aka lift) on it is much lower than when it's correcting for a gust (or actively steering).  Each fin adds drag even at zero AoA, so we prefer fewer; 3 or 4 suffice for stability or steering; so it's unlikely that anyone's bothered to estimate a heuristic for larger n.


text:
How realistic is the aviation crisis in the recent Spider-Man movie?
In the 2014 film The Amazing Spider-Man 2, there's a sequence in which a villain disables the city's power grid.  All over town, stuff loses power... including the local airport and its air traffic controllers.  This is a problem because there are currently two planes inbound, on courses that will cause them to collide with each other in midair unless the tower can warn them in time!
Luckily, Spider-Man finds a way to defeat the villain and the ATC gets their power back just in time to radio the planes to change course, and they miss each other by an extremely narrow margin.
Putting aside for the moment the question of whether or not a real ATC would have their own independent backup power source, how realistic are the two premises of this crisis?
1) That two airplanes would be inbound towards a major airport on courses that would cause them to both occupy the same position in 3D space at the same time, and
2) That neither pilot would actually notice this until either the tower told them or something suddenly goes CRUNCH!  Do commercial jet liners not have their own radar at the very least?  (Heck, my car has little radar emitters that warn me if something is too close!)

only one word: TCAS

related: What happens when the airport shuts down

If only one person in the whole airport had thought to buy a handheld radio! $217 on Amazon. Shoulda, coulda, woulda....

@AE You mean like the ones they have on charging racks in the tower for just that eventuality?

@AE Or they could run out onto the ramp and climb aboard any airplane...

This depends on the aircraft type, the flight rules and weather conditions as contributing factors.
Larger aircraft or medium sized aircraft operating under general aviation or commercial aviation will in many cases be equipped with a technology called Traffic Collision Avoidance System (TCAS) (See: How does TCAS work?), which can instruct both aircraft to take evasive action if a collision is imminent. This is especially important in Instrument Meteorological Conditions, where pilots do not rely on outside visibility, but on instrument flight only operating under Instrument Flight Rules.
In Visual Meteorological Conditions, aircraft would likely be able to see each other and take evasive manueouvers without ATC or TCAS having to warn them, e.g. this is the case for smaller aircraft operating under Visual Flight Rules.
Aircraft receive clearances and instructions to occupy a certain flight level or altitude, sometimes coupled with clearances for approaches or arrival/departure procedures. If pilots do not receive further instructions, they will start to hold over their last clearance limit and commence lost radio/comms procedures, which include holding for a given amount of time and then beginning the descent towards the airport on defined and published approaches. If the power outage is long enough, the airspace is quite busy and all other frequencies are also unavailable, one of the being the distress or guard frequency, there is a probability that two aircraft on the same level will reach their clearance limit at some point and enter a hold at the same flight level, which could result in a collision.
The scenario however assumes that really all failsafes and contingency procedures have failed.
See also ths related question: What happens when an airport shuts down?

It can happen.  The last line of defense is "See and Avoid".
Associated Press: September 04, 1987

WASHINGTON — The Federal Aviation Administration said today that it is
  working on new navigation procedures for ocean flights in the wake of
  a July incident in which a Delta Air Lines jetliner came within 30
  feet of colliding with another jet.
Investigators say the Delta airliner, flying 60 miles off course, came
  much closer to colliding with another jet than was previously thought.
The Delta plane was within Canadian air traffic control when the
  near-collision occurred, and the Canadian Aviation Safety Board said
  Thursday that it wants immediate changes in that nation's airline
  safety rules to reduce the chance of additional incidents.
However, the FAA said today that it already has a program under way to
  improve navigation procedures used by American aircraft on
  transatlantic and transpacific routes.

ATC facilities all have backup power generators.
And aircraft have TCAS.
And if one ATC facility goes down, others are out there to assume control.
Chicago Center went down several months ago due to sabotage (a fire in a closet with much of the network/switching equipment), and while it wasn't an efficient operation for some time, the other layers of ATC picked up the load and things kept working.


text:
How safe are commercial airliners from gallium infiltration?
I recently saw a video on gallium infiltration on YouTube.  Gallium is a metal with a melting point somewhere between standard room temperature and human body temperature, meaning if you pick up a block of it sitting on the table, it will melt in your hand.  If this melted metal comes into contact with solid aluminum, it will seep into it and alloy with it, decreasing the strength of the aluminum to somewhere around that of tissue paper.
When I saw this, I immediately had a horrifying thought: aren't airplanes made of aluminum?  And when I've flown in the past, I've seen a few places where there's exposed metal.  This makes me wonder if a malicious person with no regards for their own safety wouldn't be able to bring down a plane with a novelty for children that anyone can buy for a few dollars.
If someone tried to do this, is there anything in the design of modern commercial airliners that would stop them?

I have read a number of times (but only reference I could quickly find here is https://aviation.stackexchange.com/questions/62028/what-are-the-characteristics-of-the-f-16-airframe-lifetime/64022#64022 ) that there are big problems if mercury contaminates an aluminum-bodied airplane. Mercury is less common these days, but not that long ago you could buy a mercury thermometer anywhere.

There's a difference between "making a small hole in the fuselage" and "bring down a plane".

@kris Do you have a source for this "rubberized baggage compartment" claim? Not a single one I've ever seen is rubberized...

@kris Yes, because a lightweight flame resistant insulation is used, not flammable rubber.

Gallium does not interact strongly with steel so it is unlikely to damage hydraulic pipes (these are made from steel, not aluminium). Same source says it also does not interact much with polymers so unlikely to damage the insulated wiring or rubber hoses or epoxy floor panels. It may damage some sensitive point where important device is attached to the aluminium frame, but hopefully most of such places are not single points of failure.
All it can do is to make a hole in the fuselage under the very rare circumstances. Then the aircraft will be depressurized, descend and land. It is probably still not a good idea to allow gallium in the passenger compartment.

Wouldn't infiltration near the wing/body junction make it possible for the wing to snap off?

@Jeffrey You'd have to have a lot of gallium (at least kilograms, likely 10+ kg) to have any chance of penetrating all the way through to the structural members and weakening them sufficiently (that's a lot of aluminum you have to alloy). You'd have to keep it molten all the time and in direct contact with aluminum. But the worst part is that diffusion in solid metals is an exceedingly slow process, so it'd take ages to affect anything except the surface of any bigger chunk of aluminum.

Most importantly, all the aluminum in an aircraft is painted or similarly coated to protect it from corrosion. You'd first have to scrape off enough of the coating to expose a significant surface area to attack. Good luck trying to do that unnoticed during a routine passenger flight.

Awesome, thanks! All reassuring comments.

So this means it is still ok for me to fling gallium and mercury all about the passenger compartment for added entertainment when the in-flight movie is not sufficiently amusing?

@quietflyer Define "okay".


text:
What happened during this foggy landing?
Several years ago, I was flying from the USA to Buenos Aires. That's a very long flight, and it was overnight for us.  When I woke up it was early in the morning and we were over Argentina.  I don't remember exactly what the plane was, but it was a very large jet. 767 or something like that.
As we got close, the pilot came on the intercom and announced that it was too foggy down below to land, but we still had plenty of fuel reserves left and so he was going to fly in circles for a while and see if the rising sun didn't burn off some of the fog.  After about 15 minutes, he came on again and said that the fog was burning off, and we were about to begin our descent into Buenos Aires.
I was in a window seat, and I looked outside.  We went into a cloud... and never came out.  Literally the first clue I had that we were anywhere near the ground is when I felt the wheels hit the runway.  And to this day I've always wondered what in the world was going on in the cockpit.
I don't care how good the pilot's eyes were; there was zero visibility outside that airplane.  That fog we were in was the metaphorical "pea soup", and he could not possibly have landed visually.  But we came down safe, exactly where we were supposed to be, which means he must have made some sort of instrument-guided landing.  But if he was capable of making a perfect instrument landing at that runway all along, why was the fog reducing visibility ever a concern in the first place?
Does anyone here have enough experience to explain what happened that morning?

he asked to turn the runway lights way up, they are bright enough that you would need sunglasses to avoid being blinded

When you touched down could you see the runway lights going by?

@JayCarr: No, but I was looking out to the side, not straight down, so I might have missed them.

One small point is that there can't literally have been "zero visibility" outside the aircraft, otherwise the pilot wouldn't be able to taxi off the runway even if he had used autoland to actually land the aircraft. So clearly there was some useful visibility, even if it wasn't necessarily obvious from a passenger's viewpoint.

And of course there may well have been a car guiding the plane, a car you can't see from the passenger cabin.

I don't care how good the pilot's eyes were; there was zero visibility outside that airplane. That fog we were in was the metaphorical "pea soup", and he could not possibly have landed visually.

What you see from a passenger window has little relationship to what the pilot is looking for and can see out his windshield. The pilot is looking for lead-in lights (there's a lot of them) and a sequence flasher that creates the effect of a ball of light starting at the beginning of the lead-in lights and traveling to the runway. In low visibility the tower will typically turn them up to maximum intensity. These lights are very bright, so much so that after catching sight of them I usually asked for them to be turned down. None of this light system is visible from the passenger cabin of a large aircraft save for the fact that you might notice a glow outside as the airplane passes over them when it's dark.

the pilot came on the intercom and announced that it was too foggy down below to land, but we still had plenty of fuel reserves left and so he was going to fly in circles for a while and see if the rising sun didn't burn off some of the fog.

Another thing that the pilot can do if he has the fuel and if the sun is about to set, is to loiter until it gets darker. The runway visual range (RVR) will generally increase considerably in the dark.
There are other things at play as well. In patchy and/or blowing fog, what the RVR reads may not accurately reflect what the visibility is anyplace in the runway environment save where the RVR instrumentation is.
Occasionally, local politics can also be a factor. In the 1980s I used to fly into an airport that had as its only air service the commuter airline I was flying for. The local government and chamber of commerce very much wanted to keep that air service. The problem was fog. An unwritten accommodation was reached that took advantage of the rule that once inside the outer marker, you can continue the approach even though the RVR dropped below the minimum required. We knew that if the reported RVR was right at the minimun required when we were cleared for the approach, we might well be told after reporting passing the outer marker that it was a lesser value.
In two years of flying for that commuter, I never failed to land on the first approach at that airport and neither did most of the captains.
I suspect that in today's hypersensitive regulatory environment, this accommodation might not be possible, and I also don't know if the rule is still in force that permits you to continue an approach once inside the outer marker.

Other's have explained what ILS is, but if you were wondering what the pilot may have seen, it would be something like this.
The vertical red box on the right side indicates whether the plane is correctly following the proper slope into the runway. If the plane drifts lower, the magenta diamond would go higher and vice versa. The horizontal red box indicates whether the plane is correctly centered on the runway.
By keeping both in the middle, it allows the pilot or autopilot to preform safe landings even when visibility is not so good.
In this example it looks like the plane was landing on a runway with a direction of 155 degrees and about to touchdown with 120 meters feet off the ground. The red boxes themselves are for illustration, so they wouldn't be there in real life. Also this display varies between plane manufacturers but conceptually the same.

Image from: Wikipedia - Jean Victor Kerecz Arruda

+1 for the PFD image. However, are you sure that that altitude is in meters? Feet are used for altitudes in most countries other than China and former Soviet bloc nations.

You are probably correct actually. Looking at the altitude setting, it would make more sense for it to be set to 6,000 ft as opposed to 6,000 m

The display only varies by manufacturer by some colours being used, but the layout is the same in all aircraft with glass cockpit.

Nice supplementary information, but it is not a complete answer because doesn't address the actual question.

The system that allows planes to land in low visibility is the Instrument Landing System (ILS). Using ILS, a plane can know its position relative to the runway, either as a reference while manually flying, or to allow the autopilot to fly the approach to the runway, or even land the plane. There are different categories of ILS depending on the aircraft and airport equipment.
Each category has its own "minimums," which are the weather conditions required for the approach to be made. This includes the decision height, which is the height above the runway where they must be able to see the runway, the runway visual range (RVR), or the visibility down the runway, and visibility minimum, which is the general visibility.
Currently, the highest category in use is IIIb, which requires an RVR of 150 feet. Category IIIa requires 700 feet. Category II requires 1200 feet, and has a decision height of 100 feet.
In your case, the weather was probably below the minimums for one reason or another. After holding for a while, the weather cleared up enough to allow an ILS landing. Runway lights make it easier to see the runway from the cockpit, so from a passenger's perspective, it can seem like "zero visibility".

The ILS system does not fly the aircraft at all. ILS approaches are almost always hand flown, except for when weather minima requires that they be a coupled approach (ie, such that autoland is required). The ILS system just gives the aircraft feedback on its current position relative to the required course, and glidepath, to safely land the aircraft in the touchdown zone.

@SHAF by "almost always hand flown" do you mean only the last portion (and touchdown) of the approach? Pilots can use the autopilot to capture the ILS. True - auto-lands are rare, and most of the time the autopilot is disengaged at some altitude. But to the best of my knowledge, autopilot ILS capture is not uncommon.

IIRC, the autopilot must be disengaged at DH, so in that respect, nearly all approaches are hand flown to touchdown (my aircraft doesn't have such capabilities so we hand-fly everything anyway).  However, most guys I know prefer to fly the majority of the approach.  While I would agree that using autopilot to capture is probably more prevalent, I still suggest that most of the approach itself is probably hand flown.

But if he was capable of making a perfect instrument landing at that runway all along, why was the fog reducing visibility ever a concern in the first place?

Any technology, including the instrument landing system (ILS), can fail. Since life depends on it, there have to be methods in place to detect the failure and abort the approach. The final method for this is that the pilot must see the runway or approach lights when the aircraft descends to certain height above ground, the "decision height". The approach lights are very bright, so they can be seen significantly further than unlit objects.
The basic ILS installation, CAT I, usually has decision height 200 ft, sometimes more if there is uneven terrain on the approach path. There is corresponding minimum visibility that generally corresponds to distance from start of the approach lights at which the aircraft will descend to the decision height.
More advanced installations have monitoring circuits that verify the transmitters emit correct signal which allow using lower decision height or even no decision height for CAT III. The highest category, CAT IIIc, would theoretically allow zero visibility landing, but that would also require instrument taxi guidance that does not exist. So in practice there is always minimum visibility defined so the pilot can visually check correct braking, turn off and taxi. At Buenos Aires it appears currently the best available approach requires 200 m visibility. Could have been more few years back; I don't know when they last upgraded.

Aircraft can be certified to fly coupled approaches all the way to touchdown, and even landing roll out.  CATIII ILS approaches, coupled with auto land, allow the computer to fly the jet all the way to the ground, while the pilot just monitors the systems and makes sure that nothing out of the ordinary is happening.  If the weather is below certain minimums then the pilot is required to use this auto land function, and cannot manually fly the aircraft.
The pilot either used autoland, if the runway was certified for its use, or they waited until just enough fog burned off that they could legally shoot an ILS.  If the visibility really was as bad as you say though, then the pilot would have had to use autoland.


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What are the limiting factors when flying a model UAV plane to high altitude?
Setting aside the likely legal factors for a moment, what are the limiting factors when flying a model plane to high altitude?
As I see it these could be:

Battery power consumption - Model planes seem to have a battery life of less than an hour. There is a large distance to be covered in a climb ~20KM.
External forces - Strong winds/turbulence acting on a relatively small and light airframe could easily blow the plane off course. I'm not sure how severe these would be at higher altitude.
Air pressure/oxygen content - I presume this could reduce flying efficiency.
Low air temperatures affecting the operation of the electronics inside and perhaps the control surfaces.

when going for high altitude/long duration you would use a liquid fuel (tends to have better total energy storage)

What kind of engine / drive? Isn't that crucial?

Electronics tend to like it cold, hence the massive chillers in large computer rooms. Batteries, however, don't like producing electricity when they're cold, so that's an offset for an electric motored plane.

@FreeMan: Remember that while on the ground server rooms have large cooling systems on spaceships and weather balloons electronics have heaters. Electronics tend to like it around 15 degrees celsius ambient air temperature. They start failing above 50 degrees and below 5 degrees. Military grade electronics can be made to work down to -10 degrees but the temperature at 20km is around -30 degrees. Also, stuff like your iPhone and Arduino (the most common hobbyist UAV platform) are not military grade.

The same rules apply as for "real" airplanes, and the topic has been covered well in previous questions. With model airplanes, scaling laws make the task easier, rsp. the possible maximum altitude higher. I would suggest to hang the plane below a balloon to get it up without draining batteries. NASA has done it before with a remotely controlled glider.
For the limiting factors please see this question.
Here is a question for the maximum altitude of propeller aircraft.
Here is another on the feasibility of geostationary drones.
Depending on the acceptable fragility of the design, a model aircraft should easily go down to a wing loading of 10 $kg/m^2$ and then be flyable at 36 km of altitude. At a true airspeed of 155 m/s it will fly at half the speed of sound and the Reynolds number on the wing with an assumed chord of 0.25 m will be below 20,000. You need to fly it with a lift coefficient of 1.1, which will be a challenge at the low Reynolds number, and the L/D will probably be less than 10, so your propulsion needs to be quite powerful to hold the altitude up there. I haven't done the numbers, but the propeller will be huge. With back-of-the-envelope calculations, my answer for the maximum altitude is 36 km.
Turbulence should be of no concern - all flow patterns up there are of extremely large scale.
The low temperature should also be tolerable if all systems are electric. It is not so cold up there: The standard atmosphere gives -34°C.

Uhh, 36 km propellers seem to push that into the unfeasible category, right?

@PeterKämpf: The way you wrote it sounds like the propeller diameter is 36km. The "my answer is 36 km" part needs qualification if that is not what you meant.

@JanHudec: Oops - thanks for pointing this out. The sentence stood in the context of maximum altitude before, and then I added the propeller sentence ahead of it. Fixed.

@raptortech97: My bad - I added the propeller sentence, which made the next sentence look like it refers to the propeller, not the altitude, and did not get your hint. Sorry!

If you are relying on GPS to navigate that model UAV, note that some receivers have artificial limitations imposed on them due to CoCom paranoia.  The answers at the following page may be of interest:
https://electronics.stackexchange.com/questions/11868/gps-units-weather-balloon


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What is this different airplane being flown in formation by the Red Arrows
I was in Lincoln, UK today and saw the Red Arrows practicing.
After looking more closely at the photos I took, I see that one of the formation aircraft is different.

What is this different aircraft? Why are they flying two variants?

I'm fairly sure it's a Folland Gnat based on the swept wing, thing on the nose and under-wing attachments. They used to fly it (over 40 years ago). I'm still interested to know why it would be flying today?

As @Tim wrote in the comments, it's a Folland Gnat.
The Red Arrows' own website offers this explanation, in a news release posted on March 8, 2024:

A vintage jet, of the type used by the original Red Arrows, has flown alongside the present-day team to help mark their 60th display season.
The Folland Gnat, which dates from 1964, joined the Royal Air Force Aerobatic Team’s distinctive BAE Systems Hawks above the skies of Lincolnshire.
Pilots and engineers from yesteryear, as well as Air Cadets representing the future of the RAF, were also at RAF Waddington for the special diamond season celebration.

Google says it was posted 5 hours ago, so only just before I did my searches!


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Why does linear airfoil theory break down at hypersonic speeds?
Why can't we just extend linear supersonic theory beyond M = 5? I've looked at NASA papers discussing everything from geometric effects, air disassociation, and thermal coupling, but no clear and concise explanation as to why we can't just extend linear airfoil theory beyond M = 5.

The linear theory makes certain assumptions in its formulation:

The flow is irrotational and isentropic.
The perturbations are small; i.e. the bodies are small at small angles of attack.

However, hypersonic flow violates these assumptions:

For a body in hypersonic flow (blunt body with detached bow shock), an entropy gradient is developed that varies along the distance of the body. It results in an entropy layer that persists all along the body.
In a hypersonic flow, the viscous boundary layer interacts with the shock and entropy layer and the kinetic energy of the flow is dissipated by the effect of friction within the boundary layer. Due to these viscous effects, the flow is no longer irrotational.

Note that the linear theory is not applicable if the flow is considered hypersonic. M > 5 is usually taken as hypersonic. Linear theory is not applicable in transonic regime either.


text:
Can the ECS work in an unpressurized cabin?
Can you control the cabin temperature if the cabin is unpressurized? What if the "cabin" isn't really that big, like a fighter aircraft?
For example, could I cruise in comfort at 20,000 ft in an unpressurized cockpit with an oxygen mask and heater?

Yes, you could. The smaller cabin is easier to heat because of its size.
If the engine is big enough to carry you to 20,000 ft, it produces plenty of waste heat for cabin heating. When the Russians experimented with docking fighters to bombers to extend the range of those fighters, the fighter pilots complained bitterly about the freezing temperatures in their cockpits once their engine was switched off. The first prototype of the FW-190 had no armament and a shorter fuselage, so the firewall was immediately ahead of the cockpit. The test pilot said flying it felt like having his feet roasted at a fireplace because the engine heat radiated into the cockpit.
Piston-engined aircraft allow to mix pre-heated and unheated air for cabin ventilation. The heat is either from a heat exchanger or just from piping the air near the engine. Jets and turbine-powered helicopters use bleed air which has been heated in the compressor as the source of cabin heat.
This is different in gliders, because there is no heat source and when flying at high altitude it can be really cold. You can wear thick clothing, but another problem is frosting of the canopy from the inside caused by the moisture in the pilot's breath. I found this more annoying than the cold temperature when flying in freezing air.

Interesting note about waste heat from the engines. I guess I'll route a small air duct over the engines and into the cabin!

If the cabin is unpressurised, the temperature could be controlled to a limited extent- this is usually done in a number of helicopters.
You can certainly cruise at 20k feet without a pressurized cabin - WWII pilots regularly did this with supplemental oxygen and heated clothing- though I'd hardly call it comfortable.

If WWII pilots could cruise in the cold, I think our customers can do it too with today's lighter, warmer clothing.
To give you some context, I'm in a senior capstone team designing a light attack aircraft with an RFP unit cost limit of $5 million, so I'm trying to make the aircraft as bare-bones as possible!


text:
How are engines mounted onto wings?
I've heard that the engines on a commercial air transports are typically mounted with 2 or 3 bolts. Is this true? If so, how big are these bolts and what are they made of?
I'm optimizing a bolted joint for my FEA class, and I'd like to check my results against reality.
PS: The Factor of Safety on such a bolt would be nice too!

On Sunday 4 October 1992, El Al Flight 1862 crashed into a suburb of Amsterdam after the right inboard engine separated from the wing and took part of the wing and the right outboard engine with it.
The separation was caused by fatigue fracture of the fuse-pins that connect the engine to the aircraft. The engines of the Boeing 747-200 are joined to the wing by four fuse-pins.
In the investigation report you can find detailed information about the construction that mounts the engine to the wing. Especially sections 1.6.3, 1.6.4 and 1.16 are of interest to you, although I recommend reading the entire report if you have the time.

Source: Raad voor de Luchtvaart, document AAR 92-11

Source: Raad voor de Luchtvaart, document AAR 92-11

The report was a sad yet educational read.
I found information on the pins' material (4330M steel) but not on its size. I tried finding that info in the "Investigation of the outboard midspar fuse pin" report that Luchtvaart cited, but I couldn't find the other report. Any idea on how big those pins are for a 747?

Engine mounts and thrust links

How are engines mounted onto wings?

Modern engines are hanged to the pylon struts fixed to the wings, at two mounts, using an attachment device named hanger fitting in the terminology of CFMI. This is how a CFM56-7B is mounted onto the wing of a Boeing 737 NG:

CFM56-7B on the wing of a Boeing 737 NG. Adapted from CFM56-7B nacelle training manual
The hangers are shown in red, they are fixed to the engine using links (magenta) and clevises either via the fan case fitting (forward mount, green), or directly (aft mount). Then they are fixed to the pylon / strut using bolts and shear pins.
Actual parts pictures (differences are due to improvements):

Engine fan frame fitting and aft hanger for a CFM56-7B. Source
Engine mounts have varied over years. While initially the engine thrust was carried over to the wing along the engine structure, the modern method is to avoid this as it has many disadvantages. It deforms the engine structure and prevents having reduced clearances between turbine tip and walls. This in turn leads to less than optimal engine efficiency.
On modern aircraft engines are mounted at two locations: The top of the fan frame and the top of the turbine frame. No stress is borne by other components.

For additional info on frames see: In a turbofan what holds the spinning axis?

Usually the forward mount (fan frame) receives both the vertical load and the thrust load and the aft mount receives the torque load. But in some cases, when the thrust load is not desirable on the front mount, thrust links are used to transfer the thrust load to the aft mount. This is the case for the Boeing 737 NG family.
Bolts and shear pins

I've heard that the engines on a commercial air transports are
  typically mounted with 2 or 3 bolts.

It's a bit difficult to count, because there are intermediary parts between the engine and the wing. To simplify we can focus on the two mounts and how they actually transfer thrust to the strut. In this case you are close to the truth.
In our example, the forward mount uses four bolts and two pins, the aft mount uses four bolts and a pin. The thrust load is mostly borne by the shear pins while the bolts take the vertical loads (e.g. engine weight and torque) and ensure the overall positioning.
For other combinations engine/aircraft the principle is the same, parts are slightly different, e.g. this is the forward mount for a CFM56-5B on an Airbus A320:

CFM56-5B forward mount for A320. Source
There are 4 bolts and one shear pin (the other visible pin is for centering, and is used as a backup shear pin).
We can say the main force is transferred to the wing using 3 shear pins only, but don't forget the thrust links, and the bolts used to attach them, are part of the chain too.
Still 8 bolts are necessary to deal with other loads and to keep everything sticking together, and also to allow the shear pins to remain in place.
Bolts size and strength

How big are these bolts and what are they made of?

Bolts used for a CFM56-7B on a B737 NG have the part number BACB30PN14-32M in Boeing nomenclature. This bolt has a diameter of 22 mm (0.875") and is made of nickel alloy 718 a super alloy, also known as Inconel 718.
What bolts, barrel nuts and pins look like:

CFM56-7B aft mount. Source

The Factor of Safety on such a bolt would be nice too!

To have an idea of the forces involved it's good to know a Boeing 737 NG engine has a mass of about 2,000 kg, acting on the pylon in the form of a weight of 20 kN (mass x gravity acceleration). The engine generates about 100 kN of thrust. Even taking into account sudden vertical accelerations and hard landings (let's say 3 G), thrust is the main force acting on the pylon and the shear pins are more stressed than the bolts.
Nickel alloy 718 has a tensile strength of 180 N per mm², this gives each bolt a total strength of 70 kN. the 8 bolts together could sustain 560 kN, to be compared with the weight of the engine (20 kN) and thrust (100 kN).
The only event which can seemingly detach an engine from its pylon is a collision with the ground or the friction of the engine on the ground. In that case, the engine acts as a lever. The torque would break the bolts, and then the shear pins would freely escape from their holes.
Actual pictures
It's usually difficult to see the engines mounts when the engine is on the wing, as they are buried deep into the nacelle components. Click on the image to zoom.
Example:

Engine mounts on CFM56-7B. Picture by Kostas Makris. Source
Details of the mounts:

Example 2

CFM56-7B. Source
Details of the mounts:

Additional details of the CFM56-7B mounts, from the manual
Forward mount:

Forward mount. Source: CFM56-7B Training Manual (nacelle)

The hanger fitting is fixed to the strut by 4 bolts and there are two shear pins to protect the bolts from shear loads.
The hanger fitting is fixed to the fan case fitting by 3 pawl pins
The fan case fitting is fixed by 10 bolts to the fan case

Aft mount:

Aft mount. Source: CFM56-7B Training Manual (nacelle)

The hanger fitting is fixed to the strut using 4 bolts and barrel nuts, protected by a shear pin.
The hanger fitting is fixed to clevisses on the turbine frame using 4 links using  and 4 bolts.

Two thrust links transfer the thrust from the fan frame to the aft mount. There is an evener bar on the aft mount.

I think this is a much better answer than the accepted post, though it's been 4+ (!!!!) years since it was answered originally.

Awesome answer

You're correct that the engines are mounted to the aircraft using only a few bolts. Usually, the engines are mounted on the wing at three points (usually two forward and one aft) with shear (or fuse) pins, with one or two bolts at each point. The DC-10, for example 6 (later 8 pins), while the 747-200 has 6.

"DC-10 engine-pylon". Licensed under Public Domain via Wikimedia Commons.
The following figure shows the engine mounting in A350 XWB, in which the aft mounting point and the load struts can be clearly seen (the first box), in which the engine is attached with two bolts.

Image from airliners.net
The shear pins have a complex cross section and are designed in such a way that it shears off in case of an impact. For example, the following figure shows the design of (three different) shear pins used in Boeing 747.

Image from FAA overview of accident involving Israeli Airlines FLT 1862
The shear pin bolt size depends on the particular aircraft and the engine combination.
As for safety factor, they are designed to break off in case the (shear) loads on them cross a certain threshold. The engine-to-wing supporting structure is designed to release the engine when extreme forces are encountered, in order to prevent any structural damage to the wing that may impair the aircraft's ability to fly. As such, their design philosophy is different from safe load design and varies from aircraft to aircraft.
Incidents have happened where the engine has separated from the aircraft due to the large forces experienced.

Is this one of the pins!?!
http://imgur.com/PINsTMJ
If so, that is small! Any idea how long that is?

@techSultan those are rods that support the thrust reversers. The engine mounts are in the boxes to the right and left of that one, but you can't really see the pins for those.

I wonder why they need to design the shear pin to breAk when the force exceeds certain threshold. Is it required to keep the aircraft safe when it needs to have crash landing or anything else?

@dingdingdong In the event of a choice between the engine falling off or the entire wing breaking off, you want it to be the former. Planes can fly without engines but not without wings.


text:
Why do people say aviation regulations are written in blood?
I have heard it said that all aviation regulations are "written in blood". I think this means that every rule in the book exists because someone died - and there was a lesson learned. Is this accurate? Is there anything else to the story of this phrase?

No, that's all it means. Aviation regulations tend to be put in place in response to accident reports, where a plane crashed or almost crashed for lack of the regulations. So, if you're confused why a stupid-seeming regulation exists, there's a good chance that people died because of the thing the regulation changed.
Obviously, not all regulations are like that; for instance, noise limits are based on quality of life for those near airports, not crashes. However, a good number of aviation rules came into effect as recommendations following a crash report.

Not all safety-related regulations are like that either. Many are result of simply realizing something is safety-critical already during design and many are results of incident reports which is when something went wrong without causing any injury or much damage, but increased risk of accident was recognized and gets addressed.